1. Field of the Invention
The present invention relates to network devices with traffic shaping functions, as well as to bandwidth control methods using leaky bucket algorithms. More particularly, the present invention relates to a network device having bandwidth control functions to transport cells, as well as to a bandwidth control method for controlling bandwidth usage at the source end on the basis of leaky bucket algorithms.
2. Description of the Related Art
Asynchronous Transfer Mode (ATM) is a high performance, cell-based data transmission technique widely used in our communications infrastructure. ATM networks offer a mechanism to control the traffic load by limiting the rate of ingress cells flowing into the network. Such bandwidth control functions ensure that data traffic on a particular connection will not exceed the bandwidth usage limit that has previously been declared. This feature is called “Usage Parameter Control” (UPC), and the leaky bucket algorithm is commonly used to implement UPC functions.
FIG. 14 shows the concept of leaky bucket algorithm. The leaky bucket analogy refers to a bucket with a hole near the bottom that causes it to leak at a constant rate when water is poured into the bucket and the water level reaches a certain level. If the inflow rate exactly matches with the leak rate, the water level remains constant. If water is supplied at a slower or faster rate, the water level drops or rises accordingly.
The supply of water refers to incoming traffic of cells, and the water level means the amount of cells stored in a buffer. The leak from the bucket represents an average cell rate that is previously declared. The bucket depth corresponds to a tolerance parameter, and a massive burst of incoming cells could result in an overflow from the bucket. Those excessive packets would normally be discarded since they are regarded as a violation of the agreed traffic rate.
In the actual implementation of traffic policing with a leaky bucket algorithm, the leaky bucket is realized as a counter that goes up at given cell slot intervals and goes down each time a cell is sent out to the network. This counter is referred to as the leaky bucket counter.
Conventional communications techniques using leaky bucket algorithm include the one disclosed in Japanese Patent Application Publication No. 2000-83052, where paragraphs Nos. 0023 and 0024 and FIG. 3 are particularly relevant. To stabilize packet transport, the proposed packet communications system identifies characteristics of received packet traffic through measurement and provides notification of non-conforming packets, if any. More specifically, a traffic monitor with a leaky bucket counter is placed at the receiving end to detect non-conforming cells. When the count exceeds a threshold, the cells received at that time are regarded as violating MBS requirements specified in the corresponding traffic contract. The monitoring mechanism feeds this alert information back to a preceding controller to police the flow of cells. It should be noted that cells are monitored at the receiving end, not at the sending end. The problem of this control method is that it allows excessive cells to actually travel over the transmission line, thus leading to an increase in traffic load.
Because of its flexible user bandwidth control, ATM is suitable for transporting various kinds of information, including voice, video, and computer data. ATM networks support different traffic service profiles each specifying user parameters such as maximum burst size (MBS), cell delay variation tolerance (CDVT), peak cell rate (PCR), and sustainable cell rate (SCR). MBS specifies the maximum number of burst cells that can be transmitted at the PCR. CDVT gives a tolerance for the delay of cells when they are transmitted at the PCR that is specified for each particular connection. SCR denotes an average cell rate that each particular connection requires.
Conventional bandwidth control methods, however, do not use a leaky bucket mechanism in providing traffic service with MBS, CDVT, and SCR parameters. Instead, they employ a dedicated control circuit for each parameter of MBS, CDVT, and SCR. This approach increases circuit size and reduces processing speeds.